1. Field of the Invention
This invention relates to a method of efficient operation of heavy oil-fueled power generating gas turbines. The invention is also applicable to boilers that run on heavy oil, petroleum coke, residual oils from petrochemical plants, and other fuels.
2. Prior Art
In plants that generate power by means of boilers and gas turbines, a substantial portion of the operating cost is occupied by fuels and in order to reduce it, inexpensive heavy oils (including petroleum coke, asphalt and various residual oils from oil refinery) are often employed. However, heavy oils contain various corrosive compounds of elements represented by sodium (Na), sulfur (S) and vanadium (V) and this causes the problem of serious corrosive wear in hot structural members of boilers and gas turbines which come into direct contact with gases that are produced by the combustion of heavy oils. In particular, the moving blades and stationary vanes of gas turbines are subject to extreme wear for two reasons: firstly, unlike heat transfer pipes in boilers, such blades and vanes are not subjected to the strong cooling action of water vapor; secondly, recent models of gas turbines are designed to produce hotter combustion gases with a view to achieving more efficient power generation.
Various approaches have been proposed for solving this problem of hot corrosion which occurs in power generating gas turbines that are operated on heavy oils (which are hereunder called "heavy oil-burning gas turbines"). Among these approaches are:
(1) the corrosive compounds of elements such as Na, S and V contained in heavy oils are removed either physically or chemically thereby preparing less corrosive heavy oil fuels; PA0 (2) the moving blades and stationary vanes which make direct contact with hot combustion gases are formed of materials that resist the corrosive action of the combustion gases at elevated temperatures; PA0 (3) the surfaces of the moving blades and stationary vanes are protected with coatings that are highly resistant to corrosion at elevated temperatures; PA0 (4) cooling air is allowed to flow effectively through the interior of the moving blades and stationary vanes so that their surface temperatures are sufficiently lowered to retard the reaction of corrosion which occurs at elevated temperatures; and PA0 (5) a corrosion inhibitor is added to heavy oil, which is then burnt so that the inhibitor reacts chemically with the corrosive compounds in the heavy oil, thereby converting them to non-corrosive compounds.
However, these approaches have their own problems as described below and it is necessary to develop more effective and economical alternatives.
The first approach which relies upon removing corrosive components from heavy oils suffers the problem that many of the corrosive compounds are oil-soluble and cannot be separated by simple procedures; in particular, it is quite expensive and hence uneconomical to remove S and V compounds from heavy oils, although this is technically feasible.
In order to improve the hot corrosion resistance of the moving blades and stationary vanes of gas turbines in the second approach, addition of metal components such as chromium and aluminum is effective but then such additives will deteriorate the high-temperature strength and the machinability of the blades and the vanes; thus, moving blades and stationary vane parts have yet to be developed that satisfy both requirements for resistance to hot corrosion and high-temperature strength.
The third approach which relies upon protecting the surfaces of moving blades and stationary vanes with coatings that are highly resistant to corrosion at elevated temperatures has been practiced for quite many years. This method has the advantage that there is a comparatively great degree of freedom in the choice of metal components for the protective coating since it does not require the high-temperature strength needed by the blades and the vane materials but resistance to hot corrosion will suffice. This method has proved to be capable of inhibiting the occurrence of corrosion in gas turbines that use fuels of good quality which do not contain much impurities; however, no satisfactory results have been achieved with heavy oil-burning gas turbines of the type contemplated by the invention.
The fourth approach which relies upon lowering the temperatures of moving blades and stationary vanes by means of air cooling is not applicable to all situations since the inevitable use of an increased amount of the cooling air lowers the efficiency of turbine operation.
The fifth approach which burns heavy oils after addition of corrosion inhibitors is adopted most extensively with heavy oil-burning gas turbines. In this method, the Na salts in a heavy oil are removed by extraction with hot water and, thereafter, compounds of Mg, Ca, Ba, Si, etc. are added to the heavy oil, which is burnt thereby allowing the added compound to react chemically with the Na, S and V compounds in the heavy oil so that their corrosive action is inhibited.
Many corrosion inhibitors have so far been proposed for addition to heavy oils and, to mention a few, they are: oxides of Fe, Al, Zn and Sn, as well as stearates of these metals (Japanese Patent Publication No. 8430/1954); organic compounds of metals such as Co, Mn, Fe, Cu, Ca, Ba and Mg (Japanese Patent Publication No. 9663/1968); oxides such as MgO, CaO, SiO.sub.2, Cr.sub.2 O.sub.3, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 (Japanese Patent Publication No, 29283/1973, Japanese Patent Public Disclosure (Laid-Open) No. 66873/1977, and Japanese Patent No. 1,094,783); MgO and other high-melting point compounds (U.S. Pat. Nos. 2,949,008 and 3,002,825); aluminum silicate (Swiss Patent No. 314,443); and calcium silicate (U.S. Pat. No. 2,843,200).
A problem with the addition of these corrosion inhibitors is the inevitable increase in the amount of combustion residues (ash) which deposit on the surfaces of moving blades and stationary vanes. Although the corrosion inhibitors are effective in preventing the occurrence of corrosion at elevated temperatures, their deposits will narrow the channel for the passage of combustion gases and the resulting increase in draft resistance will lower the operating efficiency of gas turbines.
In Japan and other countries which depend heavily upon imported petroleum for use as fuels, the fuel price is so high that more than 85% of the cost for operating gas turbines is occupied by fuels. In this situation, it is critical to prevent the drop in the efficiency of gas turbine operation and, hence, power generation, no matter how small it will be.